1. Field of the Invention
This invention relates to an electrode comprising a support for improving strength and conductivity and an electrochemical cell therewith such as a secondary battery and an electric double-layer capacitor.
2. Description of the Related Art
There have been suggested and practically used electrochemical cells such as secondary batteries and capacitors in which a proton-conducting polymer is used as an electrode active material. FIG. 4 shows a cross section of a conventional electrochemical cell.
As shown in FIG. 4, the conventional electrochemical cell has a configuration where a cathode 6 and an anode 7 containing a proton-conducting polymer as an active material are formed on a cathodic current collector 4 and an anodic current collector 5, respectively, these electrodes are combined via a separator 8 and only protons are involved as a charge carrier. The cell is filled with an aqueous or non-aqueous solution containing a proton-donating electrolyte as an electrolytic solution, and is sealed by a gasket 9.
The cathode 6 and the anode 7 are prepared using electrode materials including an electrode active material comprising, for example, a doped or undoped proton-conducting polymer powder as a main component, a conductive auxiliary and a binder. These electrodes can be formed by (1) a method comprising the steps of placing the electrode material in a mold with a predetermined size and molding it by a hot press to form an electrode, or (2) a method comprising the steps of depositing the slurry of the electrode materials on a current collector surface by screen printing and drying the resulting film to form an electrode. Then, a cathode and an anode thus formed are mutually faced via a separator to give an electrochemical cell.
Examples of a proton-conducting compound used as an electrode active material include π-conjugated polymers such as polyaniline, polythiophene, polypyrrole, polyacetylene, poly-p-phenylene, polyphenylene-vinylene, polyperinaphthalene, polyfuran, polyflurane, polythienylene, polypyridinediyl, polyisothianaphthene, polyquinoxaline, polypyridine, polypyrimidine, polyindole, polyaminoanthraquinone, polyimidazole and their derivatives; indole π-conjugated compound such as indole trimer compounds; quinones such as benzoquinone, naphthoquinone and anthraquinone; quinone polymers such as polyanthraquinone, polynaphthoquinone and polybenzoquinone where a quinone oxygen can be converted into a hydroxyl group by conjugation; and proton-conducting polymer prepared by copolymerizing two or more of the monomers giving the above polymers. These compounds may be doped to form a redox pair for exhibiting conductivity. These compounds are appropriately selected as a cathode and an anode active materials, taking a redox potential difference into account.
Known electrolytic solutions include an aqueous electrolytic solution consisting of an aqueous acid solution and a non-aqueous electrolytic solution comprising an electrolyte in an organic solvent. In an electrode comprising a proton-conducting compound, the former aqueous electrolytic solution is frequently used because it can give a high-capacity electrochemical cell. The acid used may be an organic or inorganic acid; for example, inorganic acids such as sulfuric acid, nitric acid, hydrochloric acid, phosphoric acid, tetrafluoroboric acid, hexafluorophosphoric acid and hexafluorosilicic acid and organic acids such as saturated monocarboxylic acids, aliphatic carboxylic acids, oxycarboxylic acids, p-toluenesulfonic acid, polyvinylsulfonic acid and lauric acid.
In the conventional method (1) for forming an electrode, preparation of a thinner electrode is difficult, an electrode resistivity is higher due to limitations in its shape, an electrochemical reaction is slower and the electrode exhibits poor quick charge-discharge behavior. In the conventional method (2) for forming an electrode, preparation of a slurry giving desired properties or selection of a binder is difficult and a manufacturing process becomes more complex for preparing an electrode with good properties.
Furthermore, an electrode prepared from the above slurry has a problem that breaks, cracks or detachment may occur during drying or impregnation of an electrolytic solution, leading to poor adhesiveness to a current collector and an increased contact resistance or an electrode resistance. The electrode also has a problem that in both conventional methods (1) and (2) for forming an electrode, expansion associated with an electrochemical reaction of a proton-conducting compound with a dopant and swelling due to impregnation of an electrolytic solution cause reduction in electrode strength and breaks and cracks, leading to an increased internal resistance and deteriorated cycle properties.
Japanese Laid-open Patent Publication No. 257133/2001 (Patent Reference 1 ) has disclosed an electrode where a layer comprising particulate and/or fibrous carbon having an ion-dissociating group and an electroconductive polymer is formed on a conductive substrate and has disclosed that a surfactant in a solution in which the above carbon is dispersed by the action of the surfactant is oxidized or reduced to precipitate the carbon on the surface of the conductive substrate and the conductive polymer is formed by electrolytic polymerization. It has also described that because of simultaneous improvement in electron conductivity and ion conductivity, the electrode exhibits a higher response speed and that the electrode can be used to provide an electrochemical capacitor having a larger output density.
Japanese Laid-open Patent Publication No. 25868/2002 (Patent Reference 2) has disclosed an electric double layer capacitor where at least one of a pair of polarizable electrodes is made of a conductive polymer complexed on particulate or fibrous charcoal by electrolytic polymerization. It has described that according to the technique, conductivity can be improved, resulting in a high-capacity electric double layer capacitor allowing high-speed charge and discharge.
According to the methods disclosed in these patent references, conductivity of an electrode can be improved, and thus charge-discharge properties of an electrochemical cell comprising such an electrode can be improved. However, it leads to a complex manufacturing process and an increased production cost.
In Patent Reference 1, a conductive substrate is made of a metal such as a stainless steel as described in Examples therein. Patent Reference 2 has described that an electrode for polymerization (substrate) used in electrolytic polymerization is made of a conductor such as a metal and graphite and a conductive polymer composite formed is used as a polarized electrode as it is or after removing it from the electrode for polymerization.
As described above, the first problem in a conventional process for forming an electrode is that since a thinner electrode cannot be formed by an electrode-forming process employing press molding, an electrode resistivity is increased, leading to deteriorated response in an electrochemical reaction. The second problem is that since adhesiveness between an electrode and a current collector is inadequate or an electrode itself has poor strength according to the electrode-forming process employing a coating by screen-printing, a film electrode exhibiting good properties cannot be prepared. Furthermore, the third problem is that an electrode exhibits poor shape stability, i.e., strength during production or an electrochemical reaction.